Protecting superconducting qubits from low-frequency noise is essential for advancing superconducting quantum computation. Based on the application of a periodic drive field, we develop a protocol for engineering dynamical sweet spots, which reduce the susceptibility of a qubit to low-frequency noise. Using the framework of Floquet theory, we prove rigorously that there are manifolds of dynamical sweet spots marked by extrema in the quasienergy differences of the driven qubit. In particular, for the example of fluxonium biased slightly away from half a flux quantum, we predict an enhancement of pure dephasing by 3 orders of magnitude. Employing the Floquet eigenstates as the computational basis, we show that high-fidelity single- and two-qubit gates can be implemented while maintaining dynamical sweet-spot operation. We further confirm that qubit readout can be performed by adiabatically mapping the Floquet states back to the static qubit states, and subsequently applying standard measurement techniques. Our work provides an intuitive tool to encode quantum information in robust, time-dependent states, and may be extended to alternative architectures for quantum-information processing.

Enhancing light-matter interactions is necessary in a wide range of applications, such as sensing, nanophotonics, nonlinear and quantum optics. Nanoscale interactions are typically enhanced through localized resonances, resulting in a stringent trade-off between bandwidth, footprint, and overall enhancement factor. Here, we discuss how nonreciprocal electromagnetic hotspots, arising at the truncation of nonreciprocal and topological interfaces, can decouple these quantities, supporting extremely large and broadband field enhancements, leading to efficient nonlinear phenomena at the nanoscale and extreme broadband enhancement of local density of states. We discuss the impact of material nonlocality and surface roughness on these effects, providing closed-form expressions for the field enhancement in the presence of nonlocality and material loss, and demonstrating that many of these effects can be observed in several realistic scenarios. Finally, we outline how topological photonics can play a role in establishing these nonreciprocal hotspots. Our findings open opportunities to implement broadband nanophotonic platforms and metamaterials that exploit topological concepts to enable exotic light-matter interactions.

A cryogenic quantum dot thermometer is calibrated and operated using only a single nongalvanic gate connection. The thermometer is probed with radio-frequency reflectometry and calibrated by fitting a physical model to the phase of the reflected radio-frequency signal taken at temperatures across a small range. Thermometry of the source and drain reservoirs of the dot is then performed by fitting the calibrated physical model to new phase data. The thermometer can operate at the transition between thermally broadened and lifetime-broadened regimes and outside the temperatures used in calibration. Electron thermometry is performed at temperatures between $3.0\mathrm{K}$ and $1.0\mathrm{K}$, in both a 1-K cryostat and a dilution refrigerator. In principle, the experimental setup enables fast electron-temperature readout with a sensitivity of $4.0\pm 0.3\mathrm{mK}/\sqrt{\mathrm{Hz}}$, at kelvin temperatures. The nongalvanic calibration process gives a readout of physical parameters, such as the quantum dot lever arm. The demodulator used for reflectometry readout is readily available at relatively low cost.

Trends in photonic quantum information follow closely the technical progress in classical optics and telecommunications. In this regard, advances in multiplexing optical communications channels have also been pursued for the generation of multidimensional quantum states (qudits), since their use is advantageous for several quantum information tasks. One current path leading in this direction is through the use of space-division multiplexing multicore optical fibers, which provides a platform for efficiently controlling path-encoded qudit states. Here, we report on a parametric down-conversion source of entangled qudits that is fully based on (and therefore compatible with) state-of-the-art multicore-fiber technology. The source design uses modern multicore-fiber beam splitters to prepare the pump-laser beam as well as measure the generated entangled state, achieving high spectral brightness while providing a stable architecture. In addition, it can be readily used with any core geometry, which is crucial since widespread standards for multicore fibers in telecommunications have yet to be established. Our source represents a step toward the compatibility of quantum communications with the next-generation optical networks.

Diffractometric biosensing is a promising technology to overcome critical limitations of refractometric biosensors, the dominant class of label-free optical transducers. These limitations manifest themselves by higher noise and drifts due to insufficient rejection of refractive index fluctuations caused by variation in temperature, solvent concentration, and, most prominently, nonspecific binding. Diffractometric biosensors overcome these limitations with inherent self-referencing on the submicron scale with no compromise on resolution. Despite this highly promising attribute, the field of diffractometric biosensors has only received limited recognition. A major reason is the lack of a general quantitative analysis. This hinders comparison to other techniques and amongst different diffractometric biosensors. For refractometric biosensors, on the other hand, such a comparison is possible by means of the refractive index unit (RIU). In this paper, we suggest the coherent surface mass density, ${\mathrm{\Gamma }}_{\mathrm{coh}}$, as a quantity for label-free diffractometric biosensors with the same purpose as the RIU in refractometric sensors. It is easy to translate ${\mathrm{\Gamma }}_{\mathrm{coh}}$ to the total surface mass density ${\mathrm{\Gamma }}_{\mathrm{tot}}$, which is an important parameter for many assays. We provide a generalized framework to determine ${\mathrm{\Gamma }}_{\mathrm{coh}}$ for various diffractometric biosensing arrangements that enables quantitative comparison. Additionally, the formalism can be used to estimate background scattering in order to further optimize sensor configurations. Finally, a practical guide with important experimental considerations is given to enable readers of any background to apply the theory. Therefore, this paper provides a powerful tool for the development of diffractometric biosensors and will help the field to mature and unveil its full potential.

We study in this paper the dynamics of quantum nanoelectronic resonant tunneling diodes (RTDs) as excitable neuromorphic spike generators. We disclose the mechanisms by which the RTD creates excitable all-or-nothing spikes and we identify a regime of bursting in which the RTD emits a random number of closely packed spikes. The control of the latter is paramount for applications in event-activated neuromorphic sensing and computing. Finally, we discuss a regime of multistability in which the RTD behaves as a memory. Our results can be extended to other devices exhibiting negative differential conductance.

The nitrogen-vacancy (N-$V$) center in diamond has been established as a prime building block for quantum networks. However, scaling beyond a few network nodes is currently limited by low spin-photon entanglement rates, resulting from the N-$V$ center’s low probability of coherent photon emission and collection. Integration into a cavity can boost both values via the Purcell effect, but poor optical coherence of near-surface N-$V$ centers has so far prevented their resonant optical control, as would be required for entanglement generation. Here, we overcome this challenge, and demonstrate resonant addressing of individual, fiber-cavity-coupled N-$V$ centers, and collection of their Purcell-enhanced coherent photon emission. Utilizing off-resonant and resonant addressing protocols, we extract an enhancement of the zero-phonon line emission by a factor of up to 4, consistent with a detailed theoretical model. This model predicts that the probability of coherent photon detection per optical excitation can be increased to 10% for realistic parameters—an improvement over state-of-the art solid immersion lens collection systems by 2 orders of magnitude. The resonant operation of an improved optical interface for single coherent quantum emitters in a closed-cycle cryogenic system at $T\sim 4$ K is an important result towards extensive quantum networks with long coherence.

We show experimentally through single-shot time-resolved conductance measurements that magnetization reversal through domain-wall motion in sub-100-nm-diameter magnetic tunnel junctions is dominated by two distinct stochastic effects. The first involves the incubation delay related to domain-wall nucleation, while the second results from stochastic motion in the Walker regime. Micromagnetics simulations reveal several contributions to temporal pinning of the wall near the disk center, including vertical-Bloch-line nucleation and wall precession. We show that a reproducible ballistic motion is recovered when the Bloch and Néel wall profiles become degenerate in energy in optimally sized disks, which enables quasideterministic motion.

Ultrafast spectroscopy is an important tool for studying photoinduced dynamical processes in atoms, molecules, and nanostructures. Typically, the time to perform these experiments ranges from several minutes to hours depending on the choice of spectroscopic method. It is desirable to reduce this time overhead not only to shorten time and laboratory resources, but also to make it possible to examine fragile specimens that quickly degrade during long experiments. In this article, we motivate using compressive sensing to significantly shorten data acquisition time by reducing the total number of measurements in ultrafast spectroscopy. We apply this technique to experimental data from ultrafast transient absorption spectroscopy and ultrafast terahertz spectroscopy and show that good estimates can be obtained with as low as 15% of the total measurements, implying a sixfold reduction in the data acquisition time.

At the fundamental level, quantum communication is ultimately limited by noise. For instance, quantum signals cannot be amplified without the introduction of noise in the amplified states. Furthermore, photon loss reduces the signal-to-noise ratio, accentuating the effect of noise. Thus, most of the efforts in quantum communications have been directed towards overcoming noise to achieve longer communication distances, larger secret key rates, or to operate in noisier environmental conditions. Here, we propose and experimentally demonstrate a platform for quantum communication based on ultrafast optical techniques. In particular, our scheme enables the experimental realization of high rates and quantum signal filtering approaching a single spectrotemporal mode, resulting in a dramatic reduction in channel noise. By experimentally realizing a 1-ps optically induced temporal gate, we show that ultrafast time filtering can result in an improvement in noise tolerance by a factor of up to 1200 compared to a 2-ns electronic filter, enabling daytime quantum key distribution or quantum communication in bright fibers.

We present the design of negative-refractive-index acoustic metamaterials operating at near-megahertz frequencies, intended for the eventual aim of enabling enhanced acoustic transmission through high-impedance-contrast biological layers. Leveraging the concept of complementary acoustic metamaterials, the negative effective properties of the metamaterials are designed to match the magnitude of an ultrasound-blocking, high-impedance-contrast layer’s properties. The negative properties are obtained using a linear array of unit cells containing Helmholtz resonators and membranes. Using finite-element and analytical models, we calculate the band structure and the effective modulus and density of the proposed negative metamaterials. Using the full three-dimensional model of the metamaterials, we then simulate the enhancement of ultrasound transmission, through layers with high-impedance contrast to water. For instance, we see an improvement from 80% transmission through a high-impedance layer alone to 98% transmission through the metamaterial-plus-high-impedance layer combination. Scaling arguments are provided to estimate the system dimensions needed to provide higher operational frequencies appropriate for imaging and high-intensity ultrasound applications. Finally, as a proof of feasibility, a preliminary experimental realization of the unit-cell structure, created using the nanofabrication approach, is presented and tested in the near-megahertz regime. These results provide a step toward metamaterial-enhanced devices for noninvasive biomedical ultrasonic imaging and therapy.

We report the development of a scanning force microscope based on an ultrasensitive silicon nitride membrane optomechanical transducer. Our development is made possible by inverting the standard microscope geometry—in our instrument, the substrate is vibrating and the scanning tip is at rest. We present topography images of samples placed on the membrane surface. Our measurements demonstrate that the membrane retains an excellent force sensitivity when loaded with samples and in the presence of a scanning tip. We discuss the prospects and limitations of our instrument as a quantum-limited force sensor and imaging tool.

We theoretically investigate the generation of two entangled beams of light in the process of single-pass type-I noncollinear frequency-degenerate parametric down-conversion with an ultrashort pulsed pump. We find the spatiotemporal squeezing eigenmodes and the corresponding squeezing eigenvalues of the generated field both numerically and analytically. The analytical solution is obtained by modeling the joint spectral amplitude of the field by a Gaussian function in curvilinear coordinates. We show that this method is highly efficient and is in good agreement with the numerical solution. We also reveal that, when the total bandwidth of the generated beams is sufficiently high, the modal functions cannot be factored into spatial and temporal parts, but exhibit a spatiotemporal coupling, whose strength can be increased by shortening the pump.

We present a semiclassical model for spin-injected vertical-cavity surface-emitting lasers (spin-VCSELs) with local optical anisotropies. Particular focus is put on highly anisotropic spin lasers with broad application potential. A generalized matrix formalism for extraction of the laser modes is introduced, which enables us to calculate the spatial distribution of vectorial modes in arbitrary spin-VCSELs. The time dependence of such laser modes is further studied by means of the generalized coupled-mode theory, which is the natural anisotropic generalization of the conventional mode-decomposition approach. We use the circularly polarized optical modes as the basis for coupled-mode theory, which leads to extension of the well-known spin-flip model. In contrast to the conventional spin-flip model, the only input parameters are the geometric and local optical properties of the multilayer structure and properties of the gain media. The advantages of the theory are demonstrated in the design and optimization of spin-VCSEL structures with a high-contrast grating. We show that the proposed structures can be used for (i) polarization modulation in the terahertz range with tremendous applications for future ultrafast optical communication and (ii) as prospective compact terahertz sources.

We use computational simulations to study the electron emission and propagation in planar vacuum diodes. We show how space charge affects thermionic emission from cathodes with two different values of work function that form a checkerboard pattern of finite extent on the cathode surface. We confirm that, for intermediate cathode temperature, the local current density from low work function regions can exceed the space-charge limit for the entire cathode. As the cathode temperature rises, space-charge effects lead to homogeneous current density from the interior of the emitting area and a higher current density from its periphery. We show how beam emittance and brightness are affected, and show that the operating temperature for optimal brightness is such that it corresponds to the transition region between source-limited and space-charge-limited emissions. Finally, we show how beam current and beam quality depend on how fine grained the structure of the cathode is.

Acoustic devices play an important role in classical information processing. The slower speed and lower losses of mechanical waves enable compact and efficient elements for delaying, filtering, and storing of electric signals at radio and microwave frequencies. Discovering ways of better controlling the propagation of phonons on a chip is an important step towards enabling larger-scale phononic circuits and systems. We present a platform, inspired by decades of advances in integrated photonics, that utilizes the strong piezoelectric effect in a thin film of lithium niobate on sapphire to excite guided acoustic waves immune from leakage into the bulk due to the phononic analogue of index guiding. We demonstrate an efficient transducer matched to $50\mathrm{\Omega }$ and guiding within a $1\mu \mathrm{m}$ wide mechanical waveguide as key building blocks of this platform. Putting these components together, we realize acoustic delay lines, racetrack resonators, and meander line waveguides for sensing applications. To evaluate the promise of this platform for emerging quantum technologies, we characterize losses at low temperature and measure quality factors on the order of 50 000 at $4\mathrm{K}$. Finally, we demonstrate phononic four-wave mixing in these circuits and measure the nonlinear coefficients to provide estimates of the power needed for relevant parametric processes.

It is highly desirable to discover an electromechanical coupling that allows a dielectric material to generate curvature in response to a uniform electric field, which would help to simplify device design and reduce interface failure in designing actuators. Flexoelectricity, a two-way coupling between polarization and strain gradient, is a good candidate. However, its applications are usually limited to the nanoscale due to its inherent size dependence. Here, an inverse flexoelectret effect in silicone elastomers is introduced to overcome this limitation. On the basis of this idea, a flexing actuator that can generate large curvature at the millimeter length scale is fabricated and shown to have excellent actuation performance, comparable with that of current nanoscale flexoelectric actuators. Theoretical analysis indicates that the phenomenon originates from the interplay of electrets and Maxwell stress. This work opens an avenue for applying macroscopic flexoelectricity in actuators and flexible electronics.

Aberration-corrected electron microscopy can resolve the smallest atomic bond lengths in nature. However, the high-convergence angles that enable spectacular resolution in two dimensions have unknown three-dimensional (3D) resolution limits for all but the smallest objects ($<\sim 8$ nm). We show aberration-corrected electron tomography offers new limits for 3D imaging by measuring several focal planes at each specimen tilt. We present a theoretical foundation for aberration-corrected electron tomography by establishing analytic descriptions for resolution, sampling, object size, and dose—with direct analogy to the Crowther-Klug criterion. Remarkably, aberration-corrected scanning transmission electron tomography can measure complete 3D specimen structure of unbounded object sizes up to a specified cutoff resolution. This breaks the established Crowther limit when tilt increments are twice the convergence angle or smaller. Unprecedented 3D resolution is achievable across large objects. Atomic 3D imaging (1 Å) is allowed across extended objects larger than depth of focus (e.g., $>20$ nm) using available microscopes and modest specimen tilting ($<3^{\circ }$). Furthermore, aberration-corrected tomography follows the rule of dose fractionation where a specified total dose can be divided among tilts and defoci.

Modulating the relaxation response of magnetic nanoparticles at two distinctive frequencies can be a potential scenario to increase the spatial resolution of magnetic particle imaging (MPI), which is beneficial for biomedical research applications. Comparatively, standard MPI requires a high field gradient to acquire a sharp point-spread function to enable submillimeter imaging of magnetic nanotracers distributed within cells or small-animal models. However, this strategy may face issues in sample handling and signal processing, while decreasing the field gradient reduces the spatial resolution and causes a blurry image. To improve the MPI image quality, we introduce a high-frequency excitation field to temporally encode the Lissajous field-free-point trajectory coupled with a low-frequency magnetization response under field gradient. Unlike regular MPI, our method further decodes the moment relaxation response at narrow sidebands around the excitation frequency to obtain the spatial coordinates of magnetic nanoparticles within a field of view, instead of the magnetization harmonics induced by the driving field. Therefore, the field-free-point steering frequencies and the resulting trajectory density are critical parameters to adjust the image resolution. Furthermore, a high signal-to-noise ratio can be achieved by use of a 1-MHz excitation field even with a low amplitude. We also demonstrate two-dimensional image reconstruction of a 1-mm ring-shaped solid Resovist® sample with 0.05-mm thickness, as well as other circular and rod phantoms, placed in a $1.4\times 1.4{\mathrm{mm}}^2$ field of view under 2 T/m symmetrical field gradient on the $x$-$y$ plane and 4 T/m on the $z$ axis. Although the spatial resolution achieved appears low to differentiate two-neighboring circular phantoms of dense liquid samples, phase shifting can be further used to resolve image distortion due to the low-frequency relaxation effects.

We show experimentally that a continuous, linear, dielectric antenna in which a superluminal polarization-current distribution accelerates can be used to transmit a broadband signal that is reproduced in a comprehensible form at a chosen target distance and angle. The requirement for this exact correspondence between broadcast and received signals is that each moving point in the polarization-current distribution approaches the target at the speed of light at all times during its transit along the antenna. This results in a one-to-one correspondence between the time at which each point on the moving polarization current enters the antenna and the time at which all of the radiation emitted by this particular point during its transit through the antenna arrives simultaneously at the target. This has the effect of reproducing the desired time dependence of the original broadcast signal. For other observer-detector positions, the time dependence of the signal is scrambled, due to the nontrivial relationship between emission (retarded) time and reception time. This technique represents a contrast to conventional radio transmission methods; in most examples of the latter, signals are broadcast with little or no directivity, selectivity of reception being achieved through the use of narrow frequency bands. In place of this, the current paper uses a spread of frequencies to transmit information to a particular location; the signal is weaker and has a scrambled time dependence elsewhere. We point out the possible relevance of this mechanism to 5G neighborhood networks and pulsar astronomy.